The Mineral ULEXITE

  • Chemistry: NaCaB5O6(OH)6-5H2O, Hydrated Sodium Calcium Borate Hydroxide
  • Class: Carbonates
  • Subclass: Borates
  • Uses: an ore of boron and as mineral specimens.
  • Specimens

Ulexite, like other borates, is a structurally complex mineral. The basic structure of ulexite contains chains of sodium, water and hydroxide octahedrons linked in endless chains. The chains are linked together by calcium, water, hydroxide and oxygen polyhedra and massive boron units. The basic boron unit has a formula of B5O6(OH)6 and a charge of negative three (-3). It is composed of three borate tetrahedrons and two borate triangular groups.

Ulexite is found with the mineral borax and is directly deposited in arid regions from the evaporation of water in intermittent lakes called playas. The playas form only during rainy seasons due to runoff from nearby mountains. The runoff is rich in the element boron and is highly concentrated by evaporation in the arid climate. Eventually the concentration is so great that crystals of ulexite, borax and other boron minerals form and accumulate to great thickness.

Specimens of ulexite may form a "cotton ball" tuft of acicular crystals. These can be confused with the similar appearing tufts of the silicate mineral okenite, but are of a completely different origin with completely different mineral associations.

Ulexite is also found in a vein-like bedding habit composed of closely-packed fibrous crystals. This variety is called "TV Rock" and is popular in many rock shops, especially with children. If the specimen is approximately an inch thick and is polished flat on both sides perpendicular to the fibers, then an unusual optical phenomenon can be seen. The fibers will behave like optical fibers and transmit an image from one side of the specimen to the other. In other words, a good specimen, resting on a newspaper will have the writing appear to be on top of the specimen without any distortion of the lettering. The newspaper can easily be read upon the surface of the ulexite! This is a phenomenal optical property that will stun anyone who has not seen it before. This effect is partially the result of the large spaces in the sodium octahedral chains mentioned above.


  • Color is white or gray to colorless.
  • Luster is silky.
  • Transparency crystals are transparent to translucent.
  • Crystal System is triclinic; bar 1
  • Crystal Habits include tufts of acicular crystals called "cotton balls". Also as vein-like masses of parallel fibrous crystals.
  • Cleavage is perfect in one direction.
  • Fracture is fibrous.
  • Hardness is 2 (softer than a fingernail)
  • Specific Gravity is approximately 1.97 (very low density)
  • Streak is white.
  • Associated Minerals are borax, colemanite, hydroboracite and other borate minerals.
  • Other Characteristics: similar borate minerals have an alkaline taste, while ulexite is tasteless.
  • Notable Occurrences include several localities in California and Nevada, USA; Tarapaca, Chile and Kazakhstan.
  • Best Field Indicators are crystal habit, associations, locality, density, unique optical property, and hardness.
ULEXITE specimens:
(hover for more info)
ULEXITE specimen ule-1
$ 33.00
Dims: 2.8" x 2.1" x 1.6" (7.1 x 5.3 x 4.1 cm)
Wt: 4.33 oz. (122.7 g)
White Monster Mine, Death Valley, Calfornia, U.S.A.
This is the first Ulexite specimen that I have seen with a fibrous, radiating habit. It is still compact, but it does not occur in the highly compact, columnar "TV Rock" formation that is most well-known for this mineral. The cluster is incomplete and shows substantial damage in the form of crushed and incomplete crystals, but still shows a definite radial habit. The crystals may be individually colorless and transparent, but when compacted together show a bright, white coloration and almost complete opacity. They measure an average of just under 1" (2.5 cm) in length. The cluster is attached to a host of what I think is either trona or massive colemanite- it is difficult to tell.
no photo
ule-1 ($ 33.00)
White Monster Mine, Death Valley, Calfornia, U.S.A.
ULEXITE specimen ule-2
$ 54.00
Dims: 4.9 x 3.7 x 2.6" (12.5 x 9.4 x 6.6 cm)
Wt: 1 lb., 4.0 oz. (565 g)
Boron, Kern County, California, U.S.A.
This large cabinet specimen consists of a chunk of compact, fibrous Ulexite. It shows some fresh damage and considerable weathering, but is still in good condition. Individual crystals are impossible to study, but its habit is standard for the specie. It has a milky-white color and a silky luster due to its fibrosity, and is essentially opaque under normal light. A few other borate minerals may be present, but if so, they are difficult to isolate.
no photo
ule-2 ($ 54.00)
Boron, Kern County, California, U.S.A.
ULEXITE specimen ule-3
$ 78.00
Dims: 7.9 x 4.1 x 2.9" (20.1 x 10.3 x 7.5 cm)
Wt: 3 lbs., 1.6 oz. (1.046 kg)
Boron, Kern County, California, U.S.A.
This display piece consists of a large section of compact, columnar Ulexite. As is usual with this mineral, no definite crystals are present, but its crystalline tendencies are obvious. It has a grayish, milky-white color and a dull, silky luster along its length, and is dimly translucent. One small area shows brighter white color and fibrous, radiating tendencies. Even in its rough state, there are one or two areas that exhibit its inherent "fiber optic" properties. Only a small amount of a dull, gray-black host material is present.
no photo
ule-3 ($ 78.00)
Boron, Kern County, California, U.S.A.
ULEXITE specimen ule-4
$ 55.00
Dims:6.4x2.5x2.2" (16.3x6.4x5.6 cm)
Wt: 21.1oz. (599g)
Kern cty., California
This specimen is a columnar crystal fragment of ulexite on matrix. The ulexite crystal itself measures over 4.0" (10.1cm). It is unusual to find good matrix specimens of this mineral. The crystal is white, and very translucent on the edges where it is thinnest. This specimen shows some cleavages where it was broken away from the host material.
no photo
ule-4 ($ 55.00)
Kern cty., California


Copyright ©1995-2023 by Amethyst Galleries, Inc.